The award will be presented to Danielson on April 22 by the Mathematical Sciences Research Institute (MSRI) at the National Math Festival in Washington, DC. Danielson won the award in the Grades 3-5 category.

“For a number of years I have longed for a better shapes book,” said Danielson. “I wanted a shapes book that gives space for noticing relationships, asking questions, and thinking together,” said Danielson. “I designed Which One Doesn’t Belong? to be an invitation to a mathematical conversation.”

The book–which is intended to be used by children, parents, and teachers–features sets of four shapes with the recurring question, “which one doesn’t belong?’ Any of the shapes can be the right answer; the key is getting kids to justify their answer in their own language. The school version comes with an extensive teacher’s guide, including an “answers key” that describes one possible argument that can be made for each shape in the book. Which One Doesn’t Belong? and the teacher’s guide can both be ordered from Stenhouse.

“Which One Doesn’t Belong? encourages children to use mathematical thinking to explore new concepts,” wrote the committee who awarded the prize. “The layout is brilliant and in classroom testing, children were active readers, enthusiastic to share their insights and justifications in the discussion. Perhaps the best feature is that questions have no single, simple answer!”

Danielson has worked with math learners of all ages—12 year-olds in his former middle school classroom, Calculus students at Normandale Community College, teachers in professional development, and young children and their families at Math On-A-Stick at the Minnesota State Fair. He designs curriculum at Desmos. He is the author of Common Core Math For Parents For Dummies, the shapes book Which One Doesn’t Belong?, and the forthcoming counting book How Many? He blogs about teaching on Overthinking My Teaching, and for parents at Talking Math with Your Kids. He earned his B.A. in mathematics from Boston University, his M.A. in Education from the University of Michigan, and his Ph.D. in Mathematics Education from Michigan State University.

The Mathical Book Prize is organized by MSRI in partnership with the National Council for Teachers of English (NCTE) and the National Council for Teachers of Mathematics (NCTM).